Thank you for your reply.
I'm doing this for New Vegas, so I'm not sure if BodySlide works for that game.
There isn't much to show, to be honest...I open the helmet mesh in the program, right-click the part I want to move, click ''Transform'', then ''Edit''... a window opens with settings that move the mesh and I slightly move it backwards. After that I click ''Accept'' so the window closes. But in order for the changes to show in-game, I have to right-click on the part again, and click on ''Apply'', then save the mesh. It's really a standard procedure when moving mesh parts. Except with this one, the parts disappear in the game after being moved, for some reason.